In recent years endeavors have been made to find a nuclear fuel which is compact and produces high temperatures. Such a fuel is of particular value in a nuclear reactor for use in outer space. It is impractical to use metallic uranium for a fuel in such a reactor. This is because of its low melting point and phase changes. Alternative proposals to build stabilized fuels for fast breeder reactors have centered upon the use of uranium dioxide, uranium carbide, and uranium nitride. Also corresponding compounds of thorium, plutonium, or a combination of these elements with uranium fuels have been used.
Compatibility is a consideration involved with the selection of any nuclear fuel. The fuel itself must be compatible with the cladding material in which it is contained. The fuel must also be compatible with any materials added to it, such as refractory metals. The addition of a non-compatible element to the fuel may prevent the formation of a satisfactory cermet. For example, carbide nuclear fuels have very limited compatibility with all common materials at elevated temperatures. Another example of incompatibility is present in a composition of uranium mononitride with calcium nitride. The physical properties of this material initially appear to be within the parameters identified for this invention as being required to stabilize a uranium mononitride fuel. However, this material, unlike the nitrides of many transition metals, prevents the formation of a satisfactory cermet with uranium mononitride. Consideration of a material's thermal conductivity compatibility must also be made. Those materials that transfer heat by conduction through electrons have increased thermal conductivity and are preferred over materials that transfer heat primarily by phonon induction. Increased thermal conductivity improves the utility of a nuclear fuel.
Uranium dioxide (UO.sub.2) is a very forgiving material. Without problems of significant corrosion to cladding materials or deterioration of cermet formation, it has been shown to be compatible with stainless steels, refractory metals, and even other ceramics. It is relatively stable and easy to fabricate. It possesses a complex vapor phase and has nearly the poorest thermal conductivity of any potential nuclear fuel. It can be operated for extended periods of time, but only at low temperatures. At high temperatures its operational time is greatly decreased.
In a breeder reactor both the carbide and nitride nuclear fuels have proven greatly superior to uranium dioxide. A comparison is given in Table I in which a uranium/plutonium mix is used.
TABLE I ______________________________________ FUEL TYPE POWER KW/ft ______________________________________ UPuO.sub.2 11 UPuC 32 UPuN 44 ______________________________________
As can be seen, there is a gain in power levels over oxide fuels by a factor of approximately three for carbides and a factor of approximately four for nitrides. Carbide fuels are cheaper to make and easier to fabricate than are nitride fuels. Nitride fuels are superior to carbide fuels in two environments. The first case is when the surface temperature does not exceed 1350.degree. C. Nitrides retain fission products at these temperatures. Swelling is less than would be expected because much of it is contained in the porosity of the fuel. Carbides are not as strong. The second case is where the surface temperature exceeds 1600.degree. C. At these temperatures nitride fuel is superior, not because of swelling characteristics, but for its compatibility. Carbides have been shown to have decreased compatibility with additional elements at elevated temperatures when compared to nitrides, see, C. A. Alexander, J. J. Ward, J. S. Ogden, and C. W. Cunningham, "Carbides in Nuclear Energy", MacMillian, 190 (1964). Because it is desired to have prolonged periods of use at high temperatures in a nuclear reactor used in outer space, nitrides are the fuels disclosed in this invention.
Uranium mononitride (UN) is attractive as a nuclear reactor fuel element because of its ability to operate at high temperatures. This quality makes it attractive for use in fast reactors, especially those designed to operate in outer space. Uranium mononitride has a high uranium density and occupies approximately thirty percent less volume than uranium dioxide (UO.sub.2) at an equivalent uranium content. uranium mononitride also has a high thermal conductivity. However, a limiting factor of UN, which mitigates against its use, is its disassociation into liquid uranium and nitrogen gas under reduced pressures at higher temperatures. As is well known liquid uranium is extremely corrosive. Liquid uranium's presence within a fuel cell can cause damage and even rupturing of the cell's cladding. The presence of nitrogen gas formed upon the disassociation of uranium mononitride fuel increases the pressure within a fuel cell. As disassociation continues, an equilibrium nitrogen pressure is reached which limits the reaction. The presence of nitrogen gas within a fuel cell can cause the cell to swell.
Attempts to overcome this problem have centered upon increasing the strength of the cladding material from which the fuel cell is made, venting gases to the exterior assembly, and evenly distributing the porosity throughout the fuel material so that the fission gases formed are contained within the fuel. Still other attempts to find a dimensionally stable uranium mononitride fuel have focused upon adding additional elements to the refractory metal matrix which decrease the disassociation of uranium nitride into liquid uranium and nitrogen gas.
Examples where additional elements are mixed with uranium nitride to form a more stable fuel include: U.S. Pat. No. 4,059,539--Potter et al., in which a uranium-zirconium mononitride composition is used as a fuel and U.S. Pat. No. 3,661,709--Chubb et al., in which particles of uranium nitride and particles of a cermet of uranium nitride together with tungsten are used in a fuel. In either of these examples prolonged operating temperatures could not exceed 1650.degree. C. The upper limits of short term operating temperatures with these elements are reported to be 1700.degree. C. and 1800.degree. C., respectively.
It is an object of this invention to provide a nuclear fuel or use at high temperatures for prolonged periods of time which is dimensionally stable without significant decreases in thermal conductivity.
It is an additional object of this invention to provide the governing parameters for selection of alloys to be used with uranium mononitride to produce a dimensionally stable nuclear fuel for use at high temperatures.
It is still another object of this invention to provide a homogeneous single phase uranium mononitride composition having improved thermal stability.